Typically, drilling rigs at the surface are used to drill boreholes to reach the location of subsurface oil or gas deposits and establish fluid communication between the deposits and the surface via the borehole. Downhole drilling equipment may be directed or steered to the oil or gas deposits using well-known directional drilling techniques. The drilling equipment has a drill bit through which mud is pumped during drilling to cool the drill bit, carry away the cuttings, and maintain a pressure in the borehole greater than the fluid pressure in the subterranean formations surrounding the borehole. The drilling mud also forms a mud cake that lines the borehole.
During the drilling, it is advantageous to perform evaluations of the subterranean formations penetrated by the borehole. The drilling equipment may be removed and a wire line downhole tool deployed into the borehole to test and/or sample one or more formation fluids at various stations or positions of the wire line tool. Alternatively, the drilling equipment of a drill string may include a downhole tool to test and/or sample the fluids of the surrounding subterranean formation. The testing and/or sampling may be accomplished by a variety of formation testing tools that retrieve the formation fluids at desired borehole positions or stations, test the retrieved fluids to ensure that the retrieved fluids are substantially free of mud filtrates, and collect such fluids in one or more chambers associated with a downhole tool. The fluid samples obtained from the subterranean formations are brought to the surface and evaluated to determine the properties of the fluids and the condition of the subterranean formations, and thereby locate oil and gas deposits.
The testing and/or sampling of formation fluids has been accomplished by wire line tools or drilling equipment that include a fluid sampling probe. The fluid sampling probe may include a durable rubber pad that is mechanically pressed against the borehole wall to form a hydraulic seal. The probe may be connected to a chamber that is connected to a pump that operates to lower the pressure in the probe. When the pump lowers the pressure in the probe below the pressure of the formation fluids, the formation fluids are drawn through the probe and into the wire line or drilling equipment downhole tool to flush the formation fluids prior to the testing and/or sampling.
During the testing and/or sampling of the formation fluids, it is important that sensors in the wire line or the drilling equipment downhole tool provide accurate measurements. Typically, the formation fluids contain impurities such as, for example, drilling fluids, cuttings, mud, or different subterranean fluids. Such impurities can affect significantly the operation of the sensors of the downhole tool and result in inaccurate measurements during the testing and/or sampling of the subterranean formation fluids.